A. Technical Field
This invention relates to a novel water-absorbing agent, a production process therefor, and a water-absorbent structure.
B. Background Art
In recent years, water-absorbent resins are widely used as constituent materials of water-absorbent articles, such as disposable diapers, sanitary napkins, and so-called incontinent pads, for the purpose of causing the water-absorbent resins to absorb body fluids. Known examples of the above water-absorbent resins are disclosed in Japan Industrial Standard (JIS) K7223-1996. Examples of the known water-absorbent resins include: crosslinked products of partially neutralized polyacrylic acids; hydrolyzed products of starch-acrylic acid graft polymers, saponified products of vinyl acetate-acrylic acid ester copolymers, hydrolyzed products of acrylonitrile- or acrylamide copolymers, and crosslinked products thereof; and crosslinked polymers of cationic monomers.
The water-absorbent resins are hardly used alone because they are generally powders. They are mixed with hydrophilic cellulose fibers, such as pulverized pulps and paper, and used as water-absorbent structures such as disposable diapers (for example, U.S. Pat. No. 3,888,257). When the water-absorbent resins alone are used for absorbing much water in a short time, they cannot absorb in a short time. Therefore, the above mixing is particularly required in order to prevent the water from spreading. The cellulose fibers have functions of: retaining powdery absorbent-reins, or sufficiently spreading water to the water-absorbent resins that are distributed according to capillary action, or retaining swollen water-absorbent resins after water is absorbed.
However, when water-absorbent structures as obtained by the above art are used as disposable diapers, and when the ratio of the absorbent resin particles to the cellulose fibers is higher, the cohesive strength is weak among the cellulose fibers. Therefore, the water-absorbent structures have the following problems: the absorption properties are not as sufficient as expected because the absorbent resins as swollen are moved and dropped after they absorb urine in practical use; and the absorption structures do not display the aimed properties because the decomposition of the absorbent resin particles is caused by a certain kind of component in the urine.
Heretofore, many proposals have been made to solve the above problems. Hereinafter, their artificial contents and problems are described.
JP-A-31362/1993 discloses a method which involves: treating a surface of a water-absorbent resin particle having a carboxyl group with a crosslinking agent having two or more functional groups which can form a covalent bond by reacting with the carboxyl group in order to crosslink with a portion of the carboxyl groups; and thereafter blending the particle with a cationic polymer compound which can form an ionic bond by reacting with the carboxyl group and has a weight-average molecular weight of not less than 2,000.
Japanese Patent No. 3017584 discloses a method for producing a water-absorbent -structure, wherein the water-absorbent structure includes a water-absorbent resin particle of which the surface has an acidic group, a cellulose fiber, and a cationic polymer compound having a weight-average molecular weight of not less than 2,000.
U.S. Pat. No. 5,382,610 discloses a method which involves: treating a surface of a water-absorbent resin particle having a carboxyl group with a crosslinking agent having two or more functional groups which can form a covalent bond by reacting with the carboxyl group in order to crosslink with a portion of the carboxyl groups; and thereafter blending the particle with a cationic polymer compound which can form an ionic bond by reacting with the carboxyl group and has a weight-average molecular weight of not less than 2,000, and a method which involves producing a water-absorbent structure which comprises a water-absorbent resin particle of which the surface has an acidic group, a cellulose fiber, and a cationic polymer compound having a weight-average molecular weight of not less than 2,000.
Japanese Patent No. 3107909 discloses a particulate water-absorbing agent having the following properties: the ratio of the particles having particle diameters of not larger than 149 μm is less than 10 weight. %; and the water-absorbing agent exhibits an absorption rate of 20 to 90 seconds when 28 g of physiological saline is absorbed in the water-absorbing agent (g), with the water-absorbing agent being characterized in that: when an iron ball (a ball as described in JIS B-1501) having a diameter of 15/32inch (about 11.9 cm) freely falls from a point having a height of 20 cm to the swollen hydrogel as obtained in the above way, the iron ball bounds on the swollen hydrogel, or the swollen hydrogel is not invaded by the iron ball (about 11.9 cm) after the iron ball stands still.
JP-A-227435/1990 discloses a liquid-absorbent polymer composition, which comprises a base polymer particle having a absorption rate of not more than 20 seconds, substantially water-absorbency, and an absorption capacity of at least 30 ml/g under an ordinary pressure, and is surface-crosslinked with a multivalent ionic crosslinking agent, and is granulated so that the particle size of the polymer composition will be larger than that of the base polymer particle, with the liquid-absorbent polymer composition being characterized by having the particle distribution such that: the liquid-absorbent polymer particle does not substantially include a particle larger than 300 μm before the liquid-absorbent polymer particle is surface-crosslinked and granulated, and 40% or more of the particles have particle diameters of not larger than 150 μm.
JP-A-53550/1996 discloses a method for producing a high-water-absorbent resin, which involves: adding (a) a hydrophilic polymer and (b) a crosslinking agent into a high-water-absorbent resin hydrogel, wherein the high-water-absorbent resin hydrogel is obtained by adding 10 to 100 parts by weight of water to 100 parts by weight of a high-water-absorbent resin, wherein the hydrophilic polymer has a reactive group and the amount of the hydrophilic polymer is 0.005 to 5 parts by weight per 100 parts by weight of the high-water-absorbent resin, and wherein the crosslinking agent has two or more functional groups reactable with the hydrophilic polymer having the reactive group and the weight ratio of the hydrophilic polymer/crosslinking agent is in the range of 0.1 to 30; blending them together; and carrying out a heating reaction of them.
Water-absorbent resins as modified by these known arts are certainly difficult to move or drop when they are combined with cellulose fibers. In addition, the swollen water-absorbing agent aggregate may also have an excellent shape-maintaining property and ball burst strength (BBS). In the present invention, the “swollen water-absorbing agent aggregate” means a state such that water-absorbing agents as swollen come into contact with one another after absorbing water, and can be regarded as one lump, for example, a state such that the cohesive strength of such as ionic bond, hydrogen bond, covalent bond, and coordinate bond is applied among swollen water-absorbing agents, and the swollen water-absorbing agents come into contact with one another. However, the water-absorbent resin particles and/or water-absorbing agents do not have a sufficient absorption amount of water under a load (absorption capacity under a load (AAP)), gel layer liquid permeation rate under a load (FRUP), saline flow conductivity (SFC). Therefore, when they were combined with cellulose fibers or other materials, the absorption properties could not be said to be sufficient. For example, when the water-absorbing agents were used as a portion of a water-absorbent structure in disposable diapers, they had a low absorption capacity under a load (AAP). Therefore, they had the following serious problems: a demerit such that the urine as absorbed in the water-absorbent structure is returned to the surface of the diaper when the pressure is exerted by body weight; and a demerit such that the water-absorbent structure has a low liquid permeability therein because the gel layer liquid permeation rate under a load (FRUP) and the saline flow conductivity (SFC) are not sufficient, and therefore, the liquid does not spread enough in the water-absorbent structure, and the decrease of the absorption amount of water and the leak of the liquid are caused.
In addition, the above water-absorbent resins as modified by these known arts are certainly difficult to move or drop when they are combined with cellulose fibers. In addition, the swollen water-absorbing agent aggregate may also have an excellent shape-maintaining property and ball burst strength (BBS). However, its absorption properties are gradually lost with the passage of time after it absorbs water, and particularly there was a demerit such that the shape-maintaining property and ball burst strength (BBS) of the swollen water-absorbing agent aggregate were considerably lowered. Because these are lowered, when disposal diapers comprising a water-absorbent resin in a high ratio are especially used for a long time, it results in moving the resultant swollen gel and lowing absorption properties.
WO 97/03114 discloses a method for producing a water-absorbing agent powder, which is characterized by decreasing a residual crosslinking agent by adding a nucleophilic agent to a carboxyl-group-containing water-absorbent resin powder of which the surface neighborhood is surface-crosslinked with a crosslinking agent including an epoxy group and in which the crosslinking agent remains, wherein the water-absorbent resin powder is in a state of heated powder. In Example 1 as described in this document, an example of adding polyethylenimine as a nucleophilic agent to a water-absorbent resin powder is described.
JP-A-509591/1997 and WO 95/22356 disclose an absorbing material having an improved absorbency, which includes a mixture of: (1) two or more absorbent gel-formable particles which are water-insoluble and include a water-swellable polymer; and (2) an absorbency-improved polymer reactable with at least one component included in urine, with the water-absorbing material being characterized in that the mixture is produced by: (i) applying a solution onto the two or more absorbent gel-formable particles, wherein the solution includes an organic solvent (favorably, polar organic solvent), water, and the absorbency-improved polymer, and the weight ratio between the organic solvent and the water is at least 50:50, favorably in the range of 70:30 to 98:2; and (ii) removing a portion of the organic solvent and water from these applied absorbent gel-formable particles. In Examples as described in this document, an example of adding polyallylamine to a water-absorbent resin powder is described.
JP-A-342963/2000 discloses a method for producing an absorbing agent composition, which is characterized by adding a polyamine compound having a weight-average molecular weight of not less than 5,000 to a water-absorbent resin having an diffusive absorption capacity of not less than 25 g/g in an aqueous sodium chloride solution of 0.9 weight % after 60 minutes from the start of swelling under a load of 20 g/cm2 (1.96 kPa).
WO 96/17884 discloses a water-absorbent resin composition having a water-holding ability of not less than 20 g/g, an absorption rate of not more than 120 seconds, a liquid permeation rate of not more than 200 seconds under a load. In Example 19 as described in this document, an example of adding polyethylenimine to a water-absorbent resin particle is described.
JP-A-290000/1997 discloses an absorbing material that comprises (a) an absorbent gel particle including a water-insoluble absorbent hydrogel-formable polymer, (b) a polycationic polymer, (c) a glue fine fiber, and (d) a carrier layer, with the absorbing material being characterized in that the polycationic polymer is bonded to the absorbent gel particle and the glue fine fiber acts as an adhesive agent between the absorbent gel particle and the carrier layer.
Even in these technical arts, the cationic polymer compound as blended with the water-absorbent resin particle is not sufficiently crosslinked, or the gel layer permeation rate under, a load (FRUP) or the saline flow conductivity (SFC) of the water-absorbent resin particle is not sufficient. Therefore, the absorption properties could not be said to be sufficient in the same reason as of the above. In addition, as to the swollen water-absorbing agent aggregate, its absorption properties are gradually lost with the passage of time after it absorbs water, and there was particularly a demerit such that the shape-maintaining property and ball burst strength (BBS) of the swollen water-absorbing agent aggregate were considerably lowered.
JP-A-3123/1997 discloses a water-absorbent polymer characterized by displaying a delay bonding character in contact with a water-containing liquid. Even in this technical art, the gel layer permeation rate under a load (FRUP) or the saline flow conductivity (SFC) of a water-absorbent resin particle is not sufficient. Therefore, the absorption properties could not be said to be sufficient in the same reason as of the above. In addition, there was a demerit such that: the absorption capacity of a water-absorbing agent under a load (AAP) was not sufficient wherein the water-absorbing agent was obtained by blending the water-absorbent resin particle and a polyamine together; and the urine as absorbed in a water-absorbent structure was returned to the surface of a diaper when the pressure was exerted by body weight.
WO 97/12575 discloses a technical art to improve a gel layer permeability under a load by carrying out a reaction between a water-insoluble water-absorbing hydrogel-formable polymer and a polycationic polymer, and forming a covalent bond between both of them. However, there was a demerit such that the shape-maintaining property and ball burst strength (BBS) of a swollen water-absorbing agent aggregate were considerably lowered because the covalent bond was formed between both of them by heating. In addition, the gel layer permeation rate under a load (FRUP) or the saline flow conductivity (SFC) of a water-absorbent resin particle is not sufficient. Therefore, the absorption properties could not be said to be sufficient in the same reason as of the above.
In WO 99/34841, WO 99/34842, WO 99/34843, and WO 99/25393, an acidic-group-containing water-insoluble swellable polymer and a basic-group-containing water-insoluble swellable polymer are mixed together and used. Accordingly, when the resultant polymer comes into contact with saline, the salt in the saline is absorbed into the polymer and the respective acid group and base group are neutralized with the salt. Therefore, the following properties are sufficiently generalized: an absorption capacity under a load (AAP), a saline flow conductivity (SFC), and a shape-maintaining property and a ball burst strength (BBS) of a swollen water-absorbing agent aggregate. However, the ratio of the acidic group and the basic group is important in this technical art, and it is very uneconomical because the valuable basic water-insoluble swellable polymer must be used in a large amount (usually, the amount is nearly equal to that of the acidic-group-containing water-insoluble swellable polymer, at least not less than 10 weight %). In addition, there was a demerit such that the absorption amount of water was extremely lowered in a liquid not including salt or in the presence of more salt than the amount possible to neutralize. Furthermore, there was usually a demerit such that the free swelling capacity (GV) of the water-absorbing agent as obtained in the above technical art was not sufficient.
JP-A-95955/2000 discloses a water-absorbing agent composition comprising at least a water-absorbent resin particle having an anionic dissociative group and a water-swellable resin particle having a cationic group, with the water-absorbing agent composition being characterized in that: 45 to 90 mol % of the anionic dissociative groups of the water-absorbent resin particle are neutralized; the weight ratio (α) of the water-absorbent resin particle having the anionic dissociative group relative to the total weight of the water-absorbent resin particle having the anionic dissociative group and the water-swellable resin particle having the cationic group is at least 0.8; and the absorption capacity of the water-absorbing agent composition under a load (P) is at least 20 g/g. The water-absorbing agent as obtained by this technical art has a sufficient absorption capacity under a load (AAP) and saline flow conductivity (SFC). However, the water solubility of the water-swellable resin particle having the cationic group as used in this technical art is low. Therefore, there was a demerit such that the shape-maintaining property and ball burst strength (BBS) of a swollen water-absorbing agent aggregate were considerably lowered.